Companion Guide to Ladder Logic / ST Conversion

Are you a seasoned ladder logic veteran, but brand-new to structured text? This guide compares most of the common ladder logic structures to their equivalent structured text format.

Not every ladder logic command is available in all IDEs, so the examples draw from multiple vendors. However, any IEC 61131-compliant structured text routine will accept the ST versions.

One note about tags in ST. Just like in any PLC program, variables can exist as global or local (program) tags. Tags are never declared right beside the ladder rungs, and likewise, variables are not declared right alongside the lines of code.

In the following examples, some snippets use variables beyond the I/O tags. They are listed as suggestions, but in real code, they will be declared as local (program) scope variables in the respective IDE, usually at the very beginning of each routine.

For all of these examples, InputTag is the real-world input terminal state, and OutputTag is directly sent to an output terminal. Assume they are already declared in the global tag/variable list.

 

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Contacts and Coils

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NO Contact (XIC):

OutputTag := InputTag;

 

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NC Contact (XIO):

OutputTag := NOT InputTag;

 

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Rising Edge (One-shot Rising):

VAR    OldInput : BOOL := InputTag; END_VAR // Compare to see if it was 0 and is now 1 IF (NOT OldInput AND InputTag) THEN   OutputTag := 1; ELSE   OutputTag := 0; END_IF; // Need to replace the old value so it can compare in the next scan cycle OldInput := InputTag;

 

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Falling Edge (One-shot Falling):

VAR    OldInput : BOOL := InputTag; END_VAR // Compare to see if it was 1 and is now 0 IF (OldInput AND NOT InputTag) THEN    OutputTag := 1; ELSE    OutputTag := 0; END_IF; // Need to replace the old value so it can compare in the next scan cycle OldInput := InputTag;

 

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Momentary Output Energize (OTE):

OutputTag := NOT EstopInputTag AND (InputTag OR OutputTag);

Note: If the E-stop is a normally closed device (as most are), then use a NO contact instruction in the ladder, or remove the ‘NOT’ term from the ST code.

 

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Output Latch (OTL):

IF InputTag THEN    OutputTag := 1; END_IF; //Remember to find another spot to turn it off

 

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Output Unlatch (OTU):

IF InputTag THEN    OutputTag := 0; END_IF; //Make sure there is another spot that turns it on

 

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Toggle:

IF InputTag THEN    OutputTag := NOT OutputTag; END_IF; //This happens continuously while the rung is true. Consider adding one-shot logic to toggle on each distinct input

 

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Flasher:

VAR    OnTime : TIME := Tx; // User input ON time, written with #ms, #s, #m, or #h suffix    OffTime : TIME := Ty; // User input OFF time, written with #ms, #s, #m, or #h suffix    InitOffTime : TIME := T0#ms; // Initially turns the output ON with no delay    OldTime : TIME; END_VAR IF InputTag THEN    IF OutputTag AND ((TIME() - OldTime)>=OnTime) THEN       OutputTag := 0;       OldTime := TIME();    ELSIF NOT OutputTag AND ((TIME() - OldTime)>=InitOffTime) THEN       OutputTag := 1;       OldTime := TIME();       InitOffTime := OffTime; ELSE    OutputTag := 0; END_IF;

 

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Math

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Simple Comparison:

VAR    Threshold : INT := x; // User input limit value    Threshold2 : INT := y; // <u>Optional</u> second user input limit value    Thresholdn : INT := n; // <u>Optional</u> all other user input limit values END_VAR IF InputTag > Threshold THEN    OutputTag := 1; ELSIF InputTag > Threshold2 THEN // Optional multiple thresholds, like a fill level with intervals    MoreOutputTags := 1; ELSE    OutputTag := 0; END_IF

 

All comparison operators are valid:

X Greater than Y	X > Y
X Greater than or equal to Y	X >= Y
X Less than Y	X < Y
X Less than or equal to Y	X <= Y
X Equal to Y	X = Y
X Not equal to Y	X <> Y

 

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Comparison Limits:

VAR    LowerThreshold : INT := x; // User input limit value    UpperThreshold : INT := y; // User input limit value END_VAR IF InputTag >= LowerThreshold AND InputTag <= UpperThreshold THEN    OutputTag := 1; ELSE    OutputTag := 0; END_IF

 

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Computation:

OutputTag := InputTag + 100;

 

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Linear Scale:

As an example to help this make sense, assume you want to convert an input -10 to 10 V signal into its equivalent 4-20 mA signal. This uses the point-slope calculation of a linear equation.

$$ y = m(x-x_1)+y_1 $$

Where

$$ m = \frac{y_2-y_1}{x_2-x_1} $$

 

VAR    LowerX : INT := A; // User provided lower input value (ex. -10)    UpperX : INT := B; // User provided upper input value (ex. 10)    LowerY : INT := C; // User provided lower output value (ex. 4)    UpperY : INT := D; // User provided upper output value (ex. 20) END_VAR OutputTag := ((UpperY - LowerY)/(UpperX - LowerX))*(InputTag - LowerX) + LowerY;

 

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Find Max or Min Value:

VAR    MaxVal : INT := InputTag; END_VAR IF InputTag > MaxVal THEN    MaxVal := InputTag; END_IF

 

- OR -

 

VAR    MinVal : INT := InputTag; END_VAR IF MinVal > InputTag THEN    MinVal := InputTag; END_IF

 

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Counters and Timers

Both counters and timers are available as standard data types, accessed by function blocks in most IDEs.

Each example below will show two concepts:

1. The standard function declaration and call.
2. The code that explains the internal operation of the function, just for the love of the coding game.

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Counter (Up Count):

VAR    CounterName : CTU;    PRE : DINT := x; // User supplied preset value END_VAR CounterName(CU := InputTag, RESET := ResetInput , PV := PRE); OutputTag := CounterName.Q;

 

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VAR    ACC : DINT := 0;    PRE : DINT := x; // User supplied preset value    OldInput : BOOL := InputTag; END_VAR // Begins with a rising edge one-shot trigger    IF (NOT OldInput AND InputTag) THEN    ACC := ACC + 1; END_IF; OldInput := InputTag; // OutputTag acts as IEC 61131 Q bit IF ACC >= PRE THEN    OutputTag := 1; ELSE    OutputTag := 0; END_IF IF ResetInput THEN    ACC := 0; END_IF

 

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On-delay Timer (TON):

VAR TimerName : TON;    PRE : TIME := x; // User supplied preset value, written with #ms, #s, #m, or #h suffix END_VAR TimerName(IN := InputTag, PT := PRE); OutputTag := TimerName.Q;

 

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VAR    ACC : TIME := 0;    StartTime : TIME := TIME();    PRE : TIME := x; // User supplied preset value, written with ms, s, m, or h suffix    OldInput : BOOL := InputTag; END_VAR // Start timer on rising edge of input tag IF (NOT OldInput AND InputTag) THEN    StartTime := TIME(); END_IF OldInput := InputTag; // Continue accumulating while button is pressed IF InputTag THEN    ACC := TIME() - StartTime; ELSE    ACC := 0; // If InputTag becomes false at any time, reset ACC END_IF; // In this routine, outputTag acts as IEC 61131 Q bit IF ACC >= PRE THEN    OutputTag := 1; ELSE    OutputTag := 0; END_IF

 

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Off-delay Timer (TOF):

VAR    TimerName : TOF;    PRE : TIME := x; // User supplied preset value, written with #ms, #s, #m, or #h suffix END_VAR TimerName(IN := InputTag, PT := PRE); OutputTag := TimerName.Q;

 

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VAR    ACC : TIME := 0;    StartTime : TIME := TIME();    PRE : TIME := x; // User supplied preset value, written with ms, s, m, or h suffix    OldInput : BOOL := InputTag; END_VAR // Energize output (Q) on rising edge of input tag IF (NOT OldInput AND InputTag) THEN    OutputTag := 1; END_IF // Begin ACC time on falling edge of input tag IF (OldInput AND NOT InputTag) THEN    StartTime := TIME(); END_IF OldInput := InputTag; // Continue accumulating while button is pressed IF InputTag THEN    ACC := TIME() - StartTime; END_IF; // Reset output (Q) when time is reached IF ACC >= PRE THEN    OutputTag := 0; END_IF

 

Fellow programming friends: if you notice any errors in these codes, please point them out so they can be corrected. I tried to keep them all accurate with the HTML formatting for the page, but please flag any errors or suggest alternatives to any of these routines - we appreciate the feedback!